Kaolinite and Gibbsite Weathering of Biotite within Saprolites and Soils of Central Virginia

The mineralogical and chemical characteristics of saprolites and soils developed from granulitic, monzonitic, and charnockitic gneisses and mylonites of the Blue Ridge Mountains and western Piedmont foothills of central Virginia, were studied. Micromorphological, x‐ray diffraction (XRD), scanning electron microscopy (SEM), and microprobe analysis showed that the mineralogical and geochemical evolution of biotite in profiles developed on gneisses is consistent with pseudomorphic weathering of mica to kaolinite and halloysite, with or without a mica–vermiculite intermediate phase. On mylonitic rocks, saprolites and soils also contain multimineral pseudomorphs after biotite, in which gibbsite crystals eventually fill the whole volume. There is evidence of topotaxial formation of halloysite after biotite. Although gibbsite is present at the first stages of the weathering of biotite, it is not clear if gibbsite forms directly from the mica or if it is a weathering product of kaolinite and/or halloysite. The source of aluminium may also be located outside the original biotite crystal. These multimineral assemblages suggest that microenvironments of weathering are controlling the formation of secondary products from the parent biotite rather than the so called anti‐gibbsite effect, at least at this scale of investigation. It is suggested that the mylonitic fabric and subvertical foliation planes of these rocks are responsible for this mineralogical and geochemical evolution.